1. Field of the Invention
The present invention relates to a process for reforming hydrocarbons and naphthas to produce the most desirable aromatic hydrocarbons at enhanced yields.
2. Background of the Invention
Large quantities of aromatic chemicals and octane pool materials are produced by a hydrocarbon/naphtha reforming process. Demand in the aromatic chemicals market, particularly in the para-xylene (PX) market, has grown steadily over the past twenty years. However, demand in the octane pool market for motor gasoline has remained flat at best. As a result of this imbalance in the marketplace of aromatic product demand, the industry has experienced periods of time when strong incentives exist to produce more aromatic chemicals, particularly xylenes, but not to produce more octane pool hydrocarbons, such as benzene and toluene. Reforming units, or reformers, have some flexibility to respond to market-demand swings; however, even more flexibility is needed to control the distribution and ultimate yield of reforming products.
The purpose of any reforming process is to rearrange the molecular structure of feed hydrocarbon species, particularly with the objective of upgrading naphthas which, depending upon its prefeed treatment processing, is one or another of a complex mixture of paraffinic, naphthenic, and aromatic hydrocarbon species; which as a bulk composition has a low octane numbers to high octane numbers gasoline components. A reforming process also is used to produce aromatic chemicals. The reforming productsxe2x80x94benzene, toluene, xylenes (ortho-xylene, meta-xylene, and para-xylene), ethylbenzene, and heavy aromatics (such as mesitylene, pseudocumene, ethyltoluenes and other C9-C12 aromatics)xe2x80x94can be recovered and sold as higher value chemical raw materials, not as part of a gasoline pool.
The chemical reactions involved in a reforming process are very complex. The reactions are commonly grouped into four categories: cracking, dehydrocyclization, dehydrogenation, and isomerization. A particular hydrocarbon/naphtha feed molecule may undergo more than one category of reaction and/or may form more than one product.
Reforming reactions were first carried out in commercial units as a thermal process. With the discovery and development of several distinct and superior catalytic reforming processes, the original thermal process became obsolete in the 1960""s. Now, all reforming processes are catalyzed by either mono-functional or bi-functional reforming catalysts. A mono-functional metallic catalyst usually has only one (precious) metal catalytic sites for catalyzing the reforming reactions. Also known are bimetallic functional catalyst in which two different precious metals exist to provide two metallic catalytic sites. A bi-functional catalyst has both metal sites and acidic sites.
The selection and/or design of a particular reforming catalyst primarily depends on the hydrocarbon/naphtha feed composition, the impurities present therein, and the desired aromatic products. A catalyst can be designed, or may be selected, to favor one or more of the four categories of chemical reactions, and thereby may influence both the yield of and selectivity of conversion of paraffinic and naphthenic hydrocarbon precursors to particular aromatic hydrocarbon structures. Intensive and continuing efforts are even now being devoted to advancing reforming technology and improving the performance of reforming catalysts.
Even with the advances in catalysis for the reforming process, a need still exists to develop new and/or improved reforming processes, and duty equipment schemes, to provide the flexibility in the product-mix demanded by the world marketplace, to better use the feedstocks, and to reduce manufacturing costs.
This invention relates to a reforming process which comprises: separating a hydrocarbon feed, such as a naphtha, under first conditions effective to produce a first fraction comprising C7xe2x88x92 hydrocarbons and a second fraction comprising C8+ hydrocarbons, and thereafter separating said second C8+ fraction in a separator under second conditions effective to produce a light fraction comprising C8 and/or C8-C9 hydrocarbons and a heavy fraction comprising C9+ hydrocarbons; and reforming said light fraction in a catalytic reformer under third conditions effective to produce a reforming product within which the ultimate yield of aromatic hydrocarbon products are enhanced, and particularly as respects to the C8 aromatic hydrocarbons, the yield of xylenes is enhanced.
This invention comprises a processing technique, and a processing arrangement of duty equipment items, which provides for the concentration of those paraffinic and naphthenic hydrocarbon components in the C7-9 carbon atom number range, more preferably in the C8-9 range, and more preferably of an C8 carbon atom number, which hydrocarbon species when in such concentrated form convert under reforming conditions by contact with a reforming catalyst into C7-9 aromatic hydrocarbon structures, preferably into C8-9 aromatic hydrocarbon structures, and most preferably into xylene hydrocarbon structures, with the reforming conversion occurring with an enhanced selectivity of conversion of these paraffinic and/or naphthenic hydrocarbon precursors into such aromatic hydrocarbon structures. Recovery of these paraffinic and naphthenic precursor hydrocarbons species from the raw hydrocarbon feedstock into a so upgraded feedstock composition for the reforming reaction is maximized to the extent most practical for maximum yield production of that aromatic hydrocarbon product structure in most market demandxe2x80x94either as gasoline octane boosters (BTX) or as specialty commodity chemicals (X)xe2x80x94during their production cycle. Thus, the processing arrangement of duty equipment items herein described provides for a great flexibility in the reforming process operation in terms of singularly using as a reforming feedstock for reforming reactions fractional hydrocarbon streams produced from a raw hydrocarbon feedstock composition, or using various mixtures of such singularly produced fractional hydrocarbon streams as a feedstock for a single or a multiple reforming reaction.
Within the context of this invention, Applicants have discovered/observed as an affect thereof that (1) to exclude by a pretreatment of a C2-16 hydrocarbon feedstock, to the maximum practical extent possible C7xe2x88x92 hydrocarbon species, with a conservation within a C8+ concentrate stream prepared by such an upgrading treatment of a raw C2-16 hydrocarbon feedstock composition, of the C8 and higher carbon number hydrocarbon species constituents, aids in promoting the activity lifetime of a reforming catalyst for producing from the low octane value hydrocarbon structures therein (generally, normal, iso and napthenic hydrocarbon species) aromatic hydrocarbon structures of high octane values; (2) to then exclude from this C8+ concentrate stream essentially all C10+ hydrocarbons and essentially all C9 aromatic hydrocarbons, to the maximum practical extent possible with a conservation within a C8+ concentrate stream prepared by an upgrading treatment of the C8+ concentrate stream of C8 carbon number hydrocarbon constituents, significantly enhances the selectivity of their conversion to aromatic C8 hydrocarbon structures in comparison to aromatic hydrocarbon structures of a degraded carbon numberxe2x80x94such as benzene (a C6 aromatic) and/or toluene (a C7 aromatic)xe2x80x94while additionally enhancing production of xylenes (C8 aromatics) compared to ethylbenzene (also a C8 aromatic).
The enhancement in yield and selectivity of conversion of that quantity of C7-8 paraffinic and/or naphthenic hydrocarbon precursor into aromatic C7-9 hydrocarbons, the recovery of which precursor paraffinic and/or naphthenic hydrocarbon species into the upgrade feedstock stream for reforming is maximized to the extent practical, overall as an affect, provides for a greater total absolute yield from that quantity of precursor paraffinic/napthenic hydrocarbon initially available in the raw hydrocarbon/naphtha feedstock as recoverable aromatic hydrocarbon structuresxe2x80x94either as a mixture of BTX suitable as an octane boosting composition for an unleaded motor gasoline stock, or as single aromatic species/classes of a purity suitable for use as special commodity chemicals in the chemical production market.